FIG. 1 depicts a system called a “rectenna” (rectifier+antenna), which is generally studied for application based on the superficial waves present at the interface between two media of different characteristics, at working frequencies in the order of GHz. In Mohammad Sarehraz, “Novel Rectenna for Collection of Infrared and Visible Radiation,” University of South Florida, March 2005, AC-DC power conversion yields greater than 80% have been measured for these systems at working frequencies in the range of microwaves (GHz).
Rectenna systems attain such a high conversion efficiency by satisfying the following three conditions: a) use of directional and monochromatic sources that do not disperse EM energy in large angles and on a broad frequency band, thus allowing an effective matching between antenna and rectifier; b) use of high-power density sources that minimize losses due to the offset (threshold) of the diodes used as a rectifier; and c) use of arrays of antennas for more effectively conveying the captured EM energy to the rectifier.
Lately, systems that may still be considered rectennas have been studied as possible transducers for sensing IR electromagnetic energy by converting it into AC electric signals that may be rectified, mainly as sensors of IR images. The antennas of a bidirectional array represent as many pixels of the sensor.
In a context of working frequency corresponding to an electromagnetic (EM) wavelength in the IR range, the block called MATCHING NETWORK, differently from typical rectenna systems, does not represent (and it could not do it) a real physical circuit, but conceptually, the necessary condition of providing for an adequate power matching between the capturing antenna and the rectifier, at the working frequency (TeraHertz). A typical system for converting IR radiations into electric energy is schematically depicted in FIG. 2. The system comprises two heat sources at different temperatures and the rectenna or a bidimensional array of rectennas (as in the case of a sensor for IR images) disposed on the surface of the source at lowest temperature (heat sink).
For such a system, it has been experimentally demonstrated that the IR electromagnetic radiation exchanged between the two bodies at different temperatures has characteristics that severely limit sensitivity and effectiveness of the EM energy conversion system of the IR radiation into an electric signal. In particular: a) the IR electromagnetic radiation substantially is a low-power density radiation; b) the IR electromagnetic radiation typically has a very broad bandwidth; and c) the radiation is spatially incoherent.
FIG. 3 depicts the emission spectrum of a black body at 300 K, i.e. of very broad bandwidth, the emission peak of which is centered in correspondence of wavelength of about 10 μm. For example, considering the temperature difference between the high temperature source (T1=300 K) and the low temperature source (T2=299 K) is equal to ΔT=1K, it may be demonstrated that for the ideal case in which both sources are black bodies, the net thermal power per unit area that flows from the body at temperature T1 to the body at temperature T2 is about equal to 6 W/m2. In these conditions, the voltage on the terminals of a dipole antenna or of an array of antennas that, for example, were capable of transferring 10% of the incident power to a rectifier supposed to be optimally power matched would be of only 23 μV.
In these conditions of electromagnetic IR radiation, an efficient rectenna system may require the use of diodes capable of working at IR frequencies with an almost ideal voltage/current characteristic, that is with a practically null threshold. Moreover, in the example of an area of coherence of the net IR radiation equal to a square having a side of 10 μm, were arbitrarily and very optimistically supposed. More realistically, the incident IR radiation is not spatially coherent and this may jeopardize the possibility of using arrays of antennas that may so increase the intensity of the cumulatively captured AC signal.